Wetting is the interaction of a liquid with a solid (or another liquid) and the subsequent spreading. Wetting and non-wetting are common phenomena in nature. For example, plants have self-cleaning surfaces due to an interdependence of water repellence, surface roughness and reduced particle adhesion. Industry is also full of examples where the spreading of liquids is of paramount importance. The application of water, stain or static resistant coatings is of concern in the textile industry. Adhesion of inks and protective film coatings to polymer film products used in the photographic and electronic industries are other examples. Wetting has become key in the development of many technologies and for this reason the fundamental mechanisms involved in wetting have been investigated.
The macroscopic behaviour of a liquid is defined in terms of the contact angle (Θ). The contact angle is the tangent that the liquid (L)/vapour (V) interface makes with the solid (S) surface at the three phase contact line.
The contact angle is commonly used to quantify the wetting of a substrate. When a liquid drop is placed on a surface, the liquid contact angle will be in the range of 0° to 180°. An angle of 0° indicates complete wetting and the liquid forms a thin film over the surface of the substrate; for example, this type of wetting is observed on clean quartz surfaces.
Partial wetting occurs when the contact angle is finite and the liquid drop on the surface forms an equilibrium shape, defined by the laws of Laplace.
The contact angle can be defined using the principle of energy minimization as well as a force balance at the solid-liquid (SL), solid-vapour (SV) and liquid-vapour (LV) interfaces. The Young equation relates the contact angle to the surface energies of the three-phase line of contact. (Equation 1):
                              Cos          ⁢                                          ⁢          θ                =                                            γ              SV                        -                          γ              SL                                            γ            LV                                              (        1        )            
Surfaces that have a low energy are not readily wet by high surface energy liquids such as aqueous liquids including water. These surfaces can therefore be said to be hydrophobic. The poor aqueous wettability of the low energy surface can pose a problem when the aqueous liquid is desirably spread on the surface for some reason. The aqueous liquid may be, for example, a carrier of an active compound that is desirably delivered to the surface. The active compound can be e.g. a drug compound, an agricultural composition or a dye. Alternatively (or in addition) the aqueous liquid could be for the purpose of forming a coating over the surface of the substrate, perhaps to modify the substrate surface.
By way of example, it is desirable for dyes such as inks to spread onto paper. Furthermore, dyes for textiles made of fibres such as nylon, wool and silk preferably wet the surfaces of the fibres in order to give an even coverage by the dye on the fibre. The wetting ability of the dye bath is particularly important when dyeing nylon fibres which have been treated with a fluorochemical which renders the surface inherently hydrophobic.
In the agricultural industry, agricultural compositions are applied to flora to deliver an active compound such as a herbicide, fungicide and pesticide. Typically, the active compound is delivered in an aqueous liquid system as a foliar spray. The components of a plant such as the leaves, shoots and stalks, however, are inherently hydrophobic which means the wettability of the target surfaces by the foliar spray must be controlled in order to ensure the active compound reaches and coats the surfaces and does not just run off to top-soil.
The wetting of particle surfaces by aqueous liquids poses problems when the particles are inherently hydrophobic and/or when the void spaces between the particles prevents penetration of the liquid into the substrate. The coating of particles can be desirable when the surface chemistry of the particle is advantageously changed, for example if the particles are required to be negatively charged or positively charged.
Methods and compositions that improve the aqueous wettability of a low energy hydrophobic surface are desirable.
The aqueous liquid that is desirably spread onto a surface can be a complex liquid such as a aqueous-based glue or resin. Articles are impregnated and/or coated with resin for many reasons including to add strength, durability and/or to improve the aesthetic of the product. If individual smaller articles are coated with a resin and those articles are combined together, the resin can function as glue since it will harden upon exposure to conditions that cause the resin to cure.
The majority of all photographs produced today are produced on resin-coated photo-paper. The paper base of resin-coated photo-papers is sealed by two polyethylene layers, making it substantially impenetrable to liquids. During the sealing process, there are no chemicals or water absorbed into the paper base, so the time needed for processing, washing and drying are significantly reduced in comparison to fibre-based papers. Resin-paper prints can be finished and dried within twenty to thirty minutes and have improved dimensional stability, and furthermore do not curl upon drying.
In the panel board industry, décor papers are impregnated and coated with resin prior to lamination onto wood based panels. Initially the décor paper is impregnated with a urea formaldehyde (UF) resin by passing the paper under tension over a pre-wetting roller. A film of resin picked up by the roller is transferred to the bottom side of the paper. The resin is given some time to penetrate into the paper before being completely immersed in a resin bath of UF to wet the top-side of the paper. The next stage of the treatment process involves coating the saturated and dried UF resin impregnated paper with a melamine formaldehyde (MF) resin. This is usually done by applying the resin onto the paper using gravure rollers. The MF resin is substantially more durable than UF resin and must contiguously cover the surface. Melamine impregnated paper is ten times more impermeable than paper treated with UF resin alone.
The aim of the first stage is to fill the void spaces (pores) of the paper with relatively inexpensive UF resin solids so, that a minimal amount of the more expensive MF resin is used in the second stage. If the paper is dipped in the UF resin bath too soon after the pre-wetting rollers, a layer of air can become trapped in the core thereby preventing the paper from being adequately saturated with UF resin. If MF resin flows into voids in the paper remaining after UF resin treatment or if the MF resin does not spread sufficiently, then insufficient MF resin may remain on the surface of the paper to effectively coat it. Defects in the resultant product may occur as a result of this mechanism because the MF resin used to treat decor paper is formulated to flow just prior to full cure in order to achieve the desired textured finish on the surface of the panel. To overcome this problem it has often been necessary to add excess MF resin to the paper to ensure there will be enough on the surface to provide a good protective coating. This is expensive and can lead to longer pressing cycles with consequential lost production and possible over-cure of the coating resin.
In the laminated particle board industry individual wood particle flakes are coated with a resin in order to ensure good coverage with the resin (glue) and therefore a strong particle board as a result. The ideal particleboard flake has a high aspect ratio i.e. a high surface to volume ratio. This enables more potential contact points to bond with other flake. The corollary to this is a dust particle which has a much lower aspect ratio, analogous to a sphere. Such particles can bond to one another at only one point, irrespective of how much resin is applied. As dry flake has a very low surface free energy i.e. is a poorly wetting surface and as the resin mix has a relatively high surface tension, the interfacial energy between the two is high. This impedes the transfer and spread of the resins on the flake surface. If large flake is not effectively resinated, it could produce zones of weakness that will impact on the integrity of the resultant panel formed from the wood particle flakes.
High speed blenders, including PAL type blenders supposedly rely on “wiping” of resin from one flake to another after resin injection. To optimise blending, operators have complex models for manipulating dwell times, involving motor current to set paddle and horn angles as well as the resistance of the out-feed flaps. However, it is still the smallest flakes that have the highest resin coverage and other larger flakes are not adequately resin wet.
Accordingly, means that improve the resin wettability of low energy surfaces such as paper and wood flakes are desirable.
WO2006/127937 describes an aqueous delivery system for low energy surface structures. The low energy surface of particular interest is PTFE. The aqueous delivery system described comprises a surfactant that is added to an aqueous liquid that will ultimately be used to wet the surface. The surfactant and the aqueous liquid together form a solution to which a wetting agent is added. The wetting agent includes alcohols and mixtures of alcohols such as hexanol and octanol. The alcohol is added incrementally to the aqueous surfactant solution (e.g. Example 6, page 13, line 34). The wetting agent and surfactant are added in amounts that ensure good emulsification of the wetting agent (page 6, lines 2 to 3). Furthermore, the surfactant is chosen to be one that is able to emulsify the desired wetting agent (page 5, lines 29 to 30). The emulsion droplets are used as the delivery means for otherwise poorly aqueous insoluble compound(s).